I am not meant to have A/C...

[elsporko321]

Due to the impending summer weather, I am once again trying to get my A/C working in my 99 Tahoe.

My compressor was dead for the longestv time, but I am confident it is still alright (replaced it two years ago). About two months ago, it started cycling randomly, with no regard to what the settings on the HVAC control were (it cycled with the A/C button pushed in, but also without). I thought it was one of those deals where my karma had caught up to me and as a kindness it fixed itself. I got a manifold and some r134a and went to charge it. It cycled for a few minutes, and then just quit. I had the low pressure switch jumpered to force it to run, to pull in refrigerant, but it still cycled at its own leisure. I was able to add one can (looked like 20 psi low or so) before it quit.

I tried a jumper on all combinations of the low switch, high switch, and clutch to no avail. I am sure that the compressor is not getting power, but I wanted to make sure there wasn't anything left to test before I throw more money at a control unit (will be the third I've bought in the ten years I've had the car). I have also tried swapping the starter/ac relays, and also just jumping slots 30 & 87, but no change.

I dug up some diagrams from sunlit comet to try and trace the power output wire from the harness to the control unit, but there are 3 green wires, and none of them are light green (per the diagram).

I really love this car, but it sure does make my life difficult. Thanks again for the help!

 

[SunlitComet]

the wire is there. on the 10 wire straight plug. it is the last wire at the end #10. the #9 wire next to it is white on the control head.

Also note if you turn mode to defrost windshield settings compressor is commanded weather or not a/c button is in or out.

if your sort those relay terminal again check for volts at compressor connector if nothing there check the A/c fuse on relay block.

 

[elsporko321]

the wire is there. on the 10 wire straight plug. it is the last wire at the end #10. the #9 wire next to it is white on the control head.

Also note if you turn mode to defrost windshield settings compressor is commanded weather or not a/c button is in or out.

if your sort those relay terminal again check for volts at compressor connector if nothing there check the A/c fuse on relay block.

I went outside and tampered with it some more...I did find the green wire (#10) on the harness into the control head, but it had no power (tested w/ engine running, unplugged from control head). My defrost button doesn't stay depressed, but the light on the button comes on/goes off if I push it in far enough.

I put a jumper on the a/c relay (slots 30 and 87), the low pressure switch, and the high pressure switch. I tried any combination of Defrost, fan setting (i know these shouldn't have any impact circuit-wise, but figured what the heck), and checked the voltage at the compressor connector - nothing. 8(

The 10 amp a/c fuse in the relay box is in good shape.

 

[SunlitComet]

when compressor is commanded by control head, the head will apply 12 volts to that wire. it does no carry power to the head only from it. it sends that signal to the PCM which monitors the various sensors and makes the final determination on whether or not to activate relay. the rear window defrost is a simple non-latching button that starts a timed relay for the rear windows elements you can press it again to shut it off sooner.

frst you did not need or should jumps several things at once not everything is N.O or N.C. contacts. there is a mix of them. you could tell the pcm the pressure is very low and very high at the same time. Regardless, the two terminals I asked you to jump was supposed to be a direct feed of unswitched power. The fact that it did not happen indicates there is a broken connection somewhere on that relay block to the compressor. the fuse is hot at all times so probe its two terminals on block and see if one has power. if so there is an orange wire that goes from there to relay terminal #87 and dark green from terminal #30 to compressor that may be bad and check the compressor black ground as well. if you see now power at either fuse terminal one of the **** fuses is burned out or you have a faulty block.

 

[elsporko321]

Thanks for the reply. Just to make sure I am understanding correctly, you are saying to probe the slots on the 10a fuse under the hood (the A/C fuse), as it should have a constant 12 volts.

Is this a correct statement?:

the fuse is between the battery and the control head, and the relay is between the head and the compressor?

Is it starting to sound like my problem is a cut/shorted wire, either between the fuse box/compressor, OR, between the high pressure switch/control head? When I last tried to get my A/C working last summer, I suspected a faulty wire was the cause.

I really appreciate your help, you really are an expert when it comes to this stuff!

 

[SunlitComet]

probe the terminals the fuse goes into. or if the fuse has exposed terminals for a fuse testing you can go with that.


flow is from battery to box to comp fuse to relay on block to compressor.

you may need to just turn the block up and take a look inside the unit.

for the compressor that is.

the control head is fed from a fuse on the inside cab block for which the blowers first three speeds is also supplied from.

as for the HP switch ignore it for the moment. that relay terminal jump you did bypasses everything except the fuse to power compressor clutch. solve that before doing anything else.

 

[elsporko321]

Sounds good. When I get home from work tomorrow, I'll probe the fuse and see if I can't find a fault in the wiring to/from the fuse block. I'll post back with my results. thanks again!

 

[elsporko321]

So I just verified that I do not have 12 volts on either terminal of the 10a A/C fuse (under the hood). I see a bolt on one side of the block, and a plastic tab on the other..are those the only two things I have to worry about to pull the block up to check out the condition of the wires underneath?

 

[SunlitComet]

I believe so just be careful pulling it up in case there is something else holding it down.

 

[elsporko321]

I set out to open the fuse block, but wasn't able to open it. It was a single bolt holding it down, but it had two sets of wires going into it - 1 was a plug, but the other actually ran into the guts of the box, so there wasn't much access to the block itself, and no obvious way to remove the top.

What I did try to do was just trace all of the relevant wires to look for any cuts or breaks. Everything looked fine, with pretty much all of it still in its original protective conduit. I also checked any grounds I could see, but they all looked to be on good and tight and relatively clean.

I also tried jiggling all of the bundles of wires/connectors, with the low pressure switch jumped, to see if I could remove any short or improve any connection (if that was the problem), and kick the compressor on if only for a moment, but I had no success.

I am completely stumped on this. I would just take the thing to an A/C guy, but they are going to want to replace 1) the compressor (proven good via battery connection), 2) the faceplate (may need replaced, but no power to harness wire #10 means may not be the culprit), or 3) both.